This invention relates to a process for producing an ultrafine metal powder. More particularly, this invention relates to a process for producing an ultrafine magnetic metal powder having excellent magnetic characteristics suitable for a high density magnetic recording medium, etc., namely high coercive force and high saturation magnetization, by carrying out gas phase pyrolysis of a transition metal carbonyl compound under specific conditions.
In these days, fine particles of a .gamma.-iron oxide are usually used as a magnetic recording medium. However, these particles have coercive forces of 400 to 900 oersted and saturation magnetizations of 90 emu/g or less, and are not suitable for a high density magnetic recording medium.
A recording medium used for an 8-mm video tape recorder (8-mm VTR), a digital audio tape (DTA) or the like is required to have such magnetic characteristics as coercive force of 1300 oersted or higher and saturation magnetization of 120 emu/g or higher. If the coercive force is smaller, anti-noise characteristic will be unsatisfactory, while if the saturated magnetization is smaller, sensitivity will be undesirably inferior.
As a high density magnetic recording medium suitable for an 8-mm VTR or DTA, an ultrafine metal powder which is produced by hydrogen reduction of an iron oxide carried out in a liquid phase has been employed. However, this powder has a tendency to generate internal voids, and magnetization occurs in the voids to give a multi-pole structure, which makes a poor dispersion of the magnetic material in a magnetic paint, leading to lowering of orientation and coercive force of a magnetic tape, and the liquid phase process has such drawbacks that the apparent density of the ultrafine powder obtained is extremely low, and therefore the separation process of the ultrafine powder from the solvent used becomes difficult, that the output per solvent is suppressed low, leading to high production cost, and that since heating at a low temperature for a long time is required because the material is liable to sinter during reduction, a large scale of equipment and enormous amount of hydrogen consumption are required and besides the preparation steps are complicated. Production of ultrafine powder by means of a gas phase method, which is free from such drawbacks, is thus desired.
Methods for obtaining an ultrafine metal powder by gas phase pyrolysis of a carbonyl compound of a transition metal such as Fe, Ni, etc. have been known, as disclosed in Japanese Patent Publication Nos. 24316/1968, 11529/1969 and 31809/1977, U.S. Pat. Nos. 2,983,997 and 2,884,319.
According to the methods disclosed in these specifications, however, the particle size of the powder obtained is as large as about several microns, and no method has been successful in obtaining an ultrafine metal powder having a length of 1 micron or less and an average particle size of 0.5 micron or less as intended by the present invention. Moreover, an ultrafine metal powder having excellent magnetic characteristics of high coercive force and high saturation magnetization suitable for a high density magnetic recording medium to be used for 8-mm VTR, DAT, etc. has not been obtained.
Japanese Patent Publication Nos. 1004/1964, 16868/1970, Japanese Laid-Open Patent Publication No. 137202/1983, and U.S. Pat. Nos. 3,172,776, 3,200,007 and 3,228,882, have proposed a process for obtaining an ultrafine metal powder by carrying out pyrolytic reaction of a transition metal carbonyl compound that is dissolved in a specific solvent. However, this process is a liquid phase process having the drawbacks as mentioned above, and thus involves various problems for mass production and in economy as compared with a gas phase process.